CHICAGO (AP) — Major colleges run their football teams just like those in the NFL, relying on players to generate millions of dollars in revenue, an economist testified Wednesday before a federal agency that will decide whether Northwestern football players may form the first union for college athletes in U.S. history.

"The difference would be … the NFL pays their players," Southern Utah University sports economist David Berri told the National Labor Relations Board on the second day of a hearing in Chicago that could stretch into Friday. That colleges don't pay their football players, he said, likely boosts their programs' profitability further.

The NLRB is considering whether Wildcats' football players can be categorized under U.S. law as employees, which would give them rights to unionize. The university, the Big Ten Conference and NCAA have all maintained college players are student-athletes, not employees.

Attorneys for Northwestern began presenting their case opposing unionization, endeavoring to show that the newly formed College Athletes Players Association would provide little tangible benefit to the Northwestern players.

Union supporters say they would be able to force schools to better protect football players from head injuries. Baptiste suggested that only the NCAA, with oversight power across the country, was in position to address that.

"That has to be done on a national level," he said. "Northwestern wouldn't have control over that."

Supporters argue a union would provide athletes a vehicle to lobby for greater financial security. They contend scholarships sometimes don't even cover livings expenses for a full year.

Baptiste said NCAA rules tie Northwestern's hands, and they would bar it from assenting to demands from an on-campus football union, including calls to increase the value of scholarships. He said the NCAA caps scholarship amounts.

Berri, the economist, was called to testify on behalf of the proposed union, which is pushing the unionization bid with support from the United Steelworkers. He sought to illustrate how the relationship between Northwestern and its football players was one of employer to employees.

Profit numbers attest to the program being a commercial enterprise, he told the hearing,

Northwestern's football program reported a total profit of $76 million from 2003 to 2012, with revenues of $235 million and costs of $159 million, Berri testified. The numbers were adjusted for inflation for the private school.

Berri conceded he didn't know that maintenance of the Wildcats' stadium was not included in the expense numbers. And he said he also did not know if football profits made up for losses in other, less popular school sports.

Schools with revenue-generating football teams were in the business of entertainment, Berri said. Asked who provided those services, he responded, "Players are the ones you are watching."

Northwestern attorney Alex Barbour pressed Berri about whether he was trying to say the school exploits its football players.

"There is an economic definition of the word 'exploitation,'" he responded. "A worker is exploited … if their economic value is greater than their wages. … By that definition, they are exploited."

Barbour had said during his opening statement that allowing a college athletes' union to collectively bargain would be "a Rube Goldberg contraption that would not work in the real world" and would fundamentally change college sports.

Berri, though, pointed to the NFL and its embrace of a union, saying unionization in its case "did not cause the professional sport to collapse."

Whether the economist should have been allowed to testify was a point of contention in the morning, with Barbour complaining that Berri's analysis was irrelevant to the central question: Are college football players employees?

But after allowing the sides to debate the issue, the hearing officer overseeing the case, Joyce Hofstra, agreed to let Berri speak, saying the hearing was "novel" and she would err on the side of admitting evidence.